Pulsed Eddy Current Technique Research Articles

Crack sizing by using pulsed eddy current technique and neural Network

A neural network approach for solving an inverse problem of identification of crack width and depth is proposed. Radial Basis Function (RBF) neural networks (NN) perform the identification. It was trained using information from numerical simulated pulsed eddy current (PEC) nondestructive testing (NDT). The capability of the RBF NN was checked with information from numerical and physical experiment. The obtained results illustrate the efficiency of the approach.

Study of magnetic sensors for pulsed eddy current techniques

In this study we compare the suitability of a Hall device and a GMR sensor as the sensing device in pulsed eddy current systems. The comparative study is carried out by using both experimental results and manufacturers' data. The factors analysed are sensitivity, noise level, and operational bandwidth. Conclusions are given following the analysis of the data.

Evaluation of Methods for Detecting and Monitoring of Corrosion Damage in Risers

Offshore pipeline failure statistics have been collected for more than 30 years now and illustrate that the riser predominantly fails as a result of corrosion. The consistent wetting and drying in the splash zone combined with defects in the coatings are the usual contributors to the problem. Risers are inspected at some determined frequency and can be done by internal and external methods. Inspecting by either means brings into account caveats and limitations from the technology used as well as human factors. For example, external inspections can be inefficient and inaccurate with some tools missing defects in areas of coating disbondment. In addition, internal inspections sometimes create false positives and can miss defects. These inaccuracies in the technologies or the techniques used may miss defects that eventually lead to failure. On the other hand, using corrosion mapping and fitness-for-service (FFS) assessment from the data collected, along with the inherent conservatism of this data from limited measurement accuracy, may result in the premature replacement of risers. A literature search is being conducted to review existing riser inspection methods and identify candidate nondestructive methods for riser inspection. These methods should be capable of detecting and monitoring general corrosion, localized corrosion pitting, and stress-corrosion cracking (sulfide or hydrogen induced) as external or internal corrosion damage. Thus far, this search has found that assessing the remaining service life of aging risers is largely dependent on the accuracy of analyzing corrosion damage to the riser surface in the atmospheric, splash (tidal), submerged, and buried environmental zones. The accuracy of each technology was analyzed. The capabilities and limitations of each method/technique used for riser inspection are summarized. The investigation is focused on long- and short-range ultrasonic techniques used for initial screening and corrosion mapping. These techniques can be deployed to detect a significant reduction in wall thickness using guided and torsional waves or to map accurately a corrosion damage using single/multiple transducers and phased-array probes in manual or automated mode. A pulsed eddy-current technique that uses a stepped or pulsed input signal for the detection of corrosion areas under insulation (CUI) is also being evaluated. This allows the detection of wall-thinning areas in the riser without removing the outside coatings. In addition, it is found that filmless, real-time, and digital radiography can be used to find internal and external corrosion defects in an insulated splash zone while the riser remains in service. A survey of nondestructive evaluation (NDE) manufacturing companies, NDE inspection companies, and operating companies was completed to collect information about current instrumentation and inspection/operators’ experience for riser inspection. Examples of advanced riser inspection instrumentation and field results are included. The ability of the candidate technologies to be adapted to riser variations, the stage of standardization, and costs are also discussed.

Reduction of lift-off effects for pulsed eddy current NDT

The lift-off effect is commonly known to be one of the main obstacles for effective eddy current NDT testing as it can easily mask defect signals. Pulsed eddy current techniques, which are believed to be potentially rich of information, are also sensitive to the effect. An approach using normalisation and two reference signals to reduce the lift-off problem with pulsed eddy current techniques is proposed. Experimental testing on the proposed technique and results are presented in this report. Results show that significant reduction in the effect has been achieved mainly in metal loss and sub-surface slot inspection. The technique can also be applied for measurement of metal thickness beneath non-conductive coatings, microstructure, strain/stress measurement, where the output is sensitive to the lift-off effect.

Pulsed Eddy-Current Nondestructive Testing of Ferromagnetic Tubes

In sinusoidal eddy-current (EC) nondestructive testing (NDT) of thick ferromagnetic tubes, such as oil-well casings, average wall thickness is measured with an exciter coil and a detector coil displaced by more than two tube diameters [remote field eddy current (RFEC) technique]. Since RFEC cannot differentiate outer from inner defects, the average tube inner diameter is determined at higher frequency with another pair of coils displaced by around one tube diameter (electrical caliper). It is expected that the key drawbacks of such EC tools (several coils, complex transducer electronics, relatively high power consumption, and length) can be overcome with the application of pulse excitation. In this paper, we give a thorough analysis of the effect of the excitation frequency and the distance between the coils on the detected signal sensitivity to the tube properties, as a basis for usage of pulsed eddy current (PEC). We show that pulse excitation, thanks to its continuous frequency spectrum and time separation of direct and remote zone signals, allows simultaneous measurement of average wall thickness and inner diameter with one detector coil separated from the exciter by one to two tube diameters. The results of our experimental work confirm that PEC technique provides a significant improvement on present EC tools for NDT of ferromagnetic tubes.

Pulsed eddy current system for dynamic inspection of defects

Pulsed Eddy Current (PEC) techniques have been investigated and applied for field applications. On-line inspection is essential for many manufacturing processes and other NDE applications. This paper investigates the dynamic behaviour of pulsed eddy current techniques and the feasibility of their use in an on-line inspection system. After discussing the PEC principles and their signal signatures, an approach on improving the dynamic response of PEC is introduced and tested. The results have illustrated that the proposed PEC system and their feature extraction can be used for real-time inspection such as NDT for manufacturing processes

Conductivity tomography for non-destructive evaluation using pulsed eddy current with HTS AQUID magnetometer

In standard eddy current nondestructive evaluation, the depth of investigation is restricted to a limited range around the skin depth of the chosen excitation frequency. We developed a pulsed eddy current NDE technique which allows simultaneous analysis of the sample at all depths. The advantage of SQUID magnetometers over induction coil sensors is that the field does not decay as rapidly as its time derivative, allowing for a broader range of investigated depths. Using rectangular excitation pulse trains, eddy currents are induced in the samples. The generated magnetic response field is recorded in off-time between the transmitter pulses by an HTS SQUID magnetometer. The early time data of the transient signal corresponds to the upper layers of the sample, while the late time data deliver information from deep layers. The transient responses are averaged over a number of alternating excitation pulses in order to enhance the signal-to-noise ratio and to eliminate drift. Scan measurements of different aluminum samples are presented. From the measured data, a 3D apparent conductivity image of the sample is calculated by applying a technique known from geophysical data interpretation. From this tomographic conductivity image of the sample, the position and depth of flaws in the material is readily determined.

Pulsed eddy current transient technique with HTS SQUID magnetometer for non-destructive evaluation

Eddy current non-destructive evaluation (NDE) often uses excitation currents at a fixed frequency, which determines the depth of investigation due to the skin effect. We developed a novel pulsed eddy current NDE technique which potentially may cover a broader depth range, by analyzing the decay of the recorded transient. Eddy currents in the inspected aluminium structures were induced by turn-off of the primary magnetic field pulse in a transmitter coil. The secondary magnetic field created by the induced currents was recorded by an HTS superconducting quantum interference device magnetometer. The quickly decaying early time data of the transient signal corresponds to the information from upper layers of the plate, while the late time data delivers information from deep layers. In order to suppress environmental noise and increase the signal-to-noise ratio, repetitive square-wave pulses were used. The transient response of the plate was recorded in off-time between the transmitter pulses and averaged using a commercial LAPTEM data acquisition unit. Measurements of a crack at different depths in a stacked aluminium sample are presented. The data was analyzed by calculating the apparent conductivity of the sample as a function of depth, a technique often used in Geophysical data interpretation. Possible advantages and drawbacks of this new transient NDE technique are discussed.

Pulsed Eddy Current Technology: Characterizing Material Loss with Gap and Lift-off Variations

Ownership costs of operational aircraft have increased steadily over the years. One of the major cost drivers is structural deterioration due to corrosion. Beyond the economics, finding and characterizing corrosion is essential for the continued airworthiness of aircraft fleets. To this end, the pulsed eddy current technique holds the potential of becoming the primary means of detecting corrosion in multilayered structures. Its wide-band frequency spectrum allows the determination of a large number of parameters, such as defect size and location. Pulsed eddy current is still considered an experimental nondestructive technique because of realistic inspection problems (e.g., probe tilting, protrusion of rivets, and thickness variations in adhesive and paint) have not been addressed in the past. Recent advances change this situation and allow pulsed eddy current to be a credible field technique.

Time-Frequency Analysis of Pulsed Eddy Current Signals

Detection and measurement of the material loss due to corrosion in aircraft fuselage lap splices are of great importance to life management of ageing aircraft. Pulsed eddy current (PEC) techniques have shown the potential to fully characterise these structures. However, variations in the probe lift-off, the interlayer gaps and material thinning produce similar PEC signals, making signal interpretation very difficult. This paper describes time-frequency analysis algorithms applied to pulsed-eddy current signals from lap-joint specimens containing simulated flaws. The effect of probe lift-off, interlayer gap, and material thinning on PEC signals is examined in three-dimensional space (amplitude-time-frequency). This study shows that the time-frequency analysis of PEC signals provides specific visual patterns that can be related to the interlayer gap, lift-off, and material loss.

Pulsed eddy current signal analysis: application to the experimental detection and characterization of deep flaws in highly conductive materials

Eddy current tests in sinusoidal mode are of great interest to detect flaws in metallic structures. A limitation of this classical method concerns the detection of deep defects in very conductive materials. Such a drawback can be reduced by operating in the transient mode, using a pulsed eddy current technique. In fact, the pulsed character of the electric excitation allows a high peak value of the density of eddy currents in the material; also the broadband signal contains optimized frequencies to probe the sample over a more extended depth. The analysis of the resulting signals is then different from those issue of the sinusoidal mode. The purpose of this paper is to apply pulsed eddy current techniques to specimens containing cracks of different size and depth. First, an original signal analysis was developed to extract the characteristics of well known defects. Then, this approach has been validated by probing different samples with real flaws.

Pulsed Eddy-Current Measurements of Corrosion and Cracking in Aging Aircraft

ABSTRACTPulsed, or transient eddy-current methods are an effective tool for quantitative characterization of hidden corrosion and cracking in multi-layer aircraft structures. Eddy currents are the method of choice for this task, since they penetrate multiple layers of metal, whether or not the layers are mechanically bonded. The pulsed eddy-current technique is an important advance over conventional eddy-current methods because it rapidly acquires data over a wide range of frequencies, thereby providing more information than a conventional, single-frequency eddy-current instrument. We have combined a pulsed eddy-current instrument with a portable two-axis scanner to produce an instrument capable of rapidly scanning aircraft lap splices in situ, producing pseudo-color images that reveal hidden corrosion or cracking. A unique feature of time-domain eddy-current data is the ability to selectively filter clutter from the image by time-gating the pulsed signal. Time-gating permits the user to select the inspection depth, thereby eliminating interference from upper layers, air gaps, lift-off variation and fasteners. By using a theoretical model of the pulsed eddy-current system, it is possible to interpret the data quantitatively, yielding quantitative maps of corrosion damage. Some of the same advantages of the pulsed eddycurrent technique apply to the characterization of hidden fatigue cracks as well, although the tieory for crack signals is less advanced.

Finite element modelling of pulsed eddy currents for nondestructive testing

The pulsed eddy current technique is widely used to detect flaws in conducting plates. Time transient data from the system can be used to produce a frequency response which enables a user to detect the presence of defects. This paper is concerned with modeling this technique using 3D finite elements. Data from the time transient finite element model is used to produce a frequency response, this is compared to values from an experimental test rig. A method of analyzing the response signal in real time is also discussed. This allows a fast method of detecting and quantifying defects in conducting structures.

Application of a signal reconstruction method to evaluate pulsed eddy-current signals

To evaluate subsurface material defects, a digital signal processing system has been developed that is matched to the particular requirements of pulsed eddy-current techniques. The signal processing is designed to obtain information about defect depth within thick components or the residual wall thickness of corroded tubes or austenitic container walls. It is found that it is possible to reconstruct the pulse response signal from 8 to 10 sampled and stored amplitude values. The zerocrossing point of the signal, which is a measure of the defect depth, is relatively simple to determine from an analytical function.

Inductive TDR Measurement of Thickness of Conductive Plating on Wire

A pulsed eddy current technique is described that is suitable for plating thickness measurements of low-permeability metals such as Au or Ag on high-permeability wire in the 3-80 mg/ in' range. Equipment costs, speed, and accuracy are such as to make this technique a strong contender not only for laboratory measurements but also for production line testing. The method is usable for plating thickness measurements whenever the ratio of relative permeability to electrical conductivity for the plating material differs substantially from that for the substrate, with the added condition that at least the plating is electrically conductive.